It is known in the art to use a ballast circuit to heat the two filaments of a fluorescent bulb to a high temperature, such that when an electric field is applied between the filaments, they emit electrons and ionize the gas in the bulb. Responsive to radiation generated due to the electric current flowing through the gas, phosphors coating the inner surface of the bulb fluoresce, emitting visible light. The ballast typically controls both the initial ignition and the steady-state operation of the bulb.
U.S. Pat. No. 5,021,714 to Swanson et al., whose disclosure is incorporated herein by reference, describes a circuit for starting and operating fluorescent bulbs from an AC low-frequency power source. A ballast generates a voltage, whose frequencies include a plurality of harmonics of the power-source frequency, which voltage causes a capacitor and a cathode heating transformer to resonate responsive to the harmonics. The resonant voltage is applied across the fluorescent bulbs to aid the starting of their discharge, and thereafter the bulbs operate at the AC power source frequency.
U.S. Pat. No. 5,723,953 to Nerone et al., whose disclosure is incorporated herein by reference, discloses a high voltage gas discharge lamp ballast, including a resonant load circuit which incorporates the lamp, and includes two resonant impedances whose values determine the operating frequency of the resonant load circuit. High voltage switches are used to disconnect the lamp's filaments during the pre-heating phase.
U.S. Pat. No. 5,208,511 to Garbowicz, whose disclosure is incorporated herein by reference, describes a fluorescent lamp system which includes a ballast with primary and secondary windings and a switch for each electrode of each of the lamps in the lamp system. Each switch operates in response to the voltage across its associated lamp, such that after the lamp turns on, the switch interrupts the connection of its associated electrode to a heater winding.
Additionally, U.S. Pat. No. 5,015,923 to Nilssen, U.S. Pat. No. 5,563,473 to Mattas et al., and U.S. Pat. No. 5,677,602 to Paul et al., whose disclosures are incorporated herein by reference, describe other electronic ballasts for use with fluorescent bulbs.